The present invention relates to a sheet ejection device having a shift processing function, an image forming apparatus equipped with such a sheet ejection device, and a post-processing apparatus provided with such a sheet ejection device.
The sheet processing apparatus for processing a great number of sheets is often provided with a sheet ejection device having a function of shifting the position for each number of sheets having been set and loading the sheets on the ejection tray.
The sheet ejection device equipped with the shift function is required to ensure that each of the sheet bundles sorted out by shift processing is aligned to a high precision. For this purpose, development efforts have been made to implement a shift processing mechanism capable of providing a highly advanced aligning function.
The image forming system capable of high-speed processing which includes an image forming apparatus tends to be utilized as a quick printing apparatus. When this image forming system is used as a quick printing apparatus, the image forming system is increasingly required to ensure that the sheets having been subjected to the processing of image formation or the like are ejected while being aligned with high precision.
There has been an increasing demand for the sheet shift ejection as the sheet ejection mode.
For example, the Japanese Unexamined Patent Application Publication No. 2006-206331 proposes a method of installing a shifting mechanism on the ejection tray, whereby sheets are shifted in a highly aligned form and are stacked in position.
Referring to FIG. 1, the following describes the overview of the shifting mechanism disclosed in the Japanese Unexamined Patent Application Publication No. 2006-206331, FIG. 1 shows that sheets are aligned by a pair of aligning members 102a and 102b, and shift processing is carried out.
The aligning members 102a and 102b travel above the sheets stacked on an ejection tray, and determine the position in the direction at right angles to the sheet ejection direction.
The bottom edges of the aligning members 102a and 102b are formed in a gently curved configuration so as to ensure contact with sheets, as illustrated in FIG. 14 of the Japanese Unexamined Patent Application Publication No. 2006-206331, for example.
This configuration allows a pair of aligning members 102a and 102b to accomplish the function of setting the sheet position and the function of aligning the sheets at the set position alternately.
The shifting mechanism disclosed in the Japanese Unexamined Patent Application Publication No. 2006-206331, however, entails a problem of insufficient aligning precision, as will be described below.
The aforementioned problem will be discussed with reference to FIGS. 1 and 2.
The topmost surface of the sheet on the ejection tray is maintained at a predetermined height indicated by point P0 under the control wherein the ejection tray travels in the vertical direction using the top surface sensor for detecting the topmost surface of the sheet.
At the position wherein the alignment member SB comes in contact with the sheets, however, the height and angle of the topmost surface of the sheet are changed by the curling of sheets.
In FIG. 1, Sn indicates the topmost surface of the uncurled sheet, while Sm shows the topmost surface of the curled sheet.
As a result of changes on the sheet topmost surface as shown in FIG. 1, the alignment member changes from SBn to SBm due to curling of the sheet. This change causes the contact point between the sheet and alignment member to be changed from P1 to P2. As is apparent from the drawing, the contact point between the sheet and alignment member shifts not only in height but also in the sheet ejection direction W.
As shown in FIG. 2, based on the assumption that the sheets are placed correctly on the ejection tray, the center point of action of the alignment member is set at point P1, wherein this alignment member aligns the sheet by reciprocating motion across the sheet width at right angles to the sheet ejection direction W. The contact point of the alignment member SBn for regulating the position of the sheet Sn that does not curl is P1, and agrees with the center point of action P1 of the alignment member that performs reciprocating motion. However, the contact point P2 of the alignment member SBm for regulating the position of the curled sheet is misaligned with the center point of action P1.
Accordingly, when sheets are uncurled and are stacked correctly on the ejection tray, the contact point of the alignment member for regulating the position and the center point of action of the other alignment member for carrying out alignment operation correspond to the same point P1 in the sheet ejection direction W, and sheets are aligned to the state indicated by Sn.
However, if sheets are curled, the contact point of the position regulating alignment member shifts from P1 to P2, as shown in FIG. 1.
As a result, the contact point P2 of the alignment member for regulating the position of the sheet is misaligned with the center point of action P1 of the other alignment member for carrying out alignment operation, in the sheet ejection direction W, as shown in FIG. 2.
Because of this misalignment, the force of the alignment member for carrying out alignment operation acts as the moment for rotating the sheet, so that sheets are inclined, as indicated by Sm of FIG. 2.
Specifically, correct alignment of sheets is not achieved.